Complementary sensory and associative microcircuitry in primary olfactory cortex

Lateral entorhinal cortex afferents reconfigure the activity in piriform cortex circuits

Odors are key signals for guiding spatial behaviors such as foraging and navigation in rodents. Recent findings reveal that odor representations in the piriform cortex (PCx) also encode spatial context information. However, the brain origins of this information and its integration into PCx microcircuitry remain unclear. This study investigates the lateral entorhinal cortex (LEC) as a potential source of spatial contextual information affecting the PCx microcircuit and its olfactory responses. Using mice brain slices, we performed patch-clamp recordings on superficial (SP) and deep (DP) pyramidal neurons, as well as parvalbumin (PV) and somatostatin (SOM) inhibitory interneurons. Concurrently, we optogenetically stimulated excitatory LEC projections to observe their impact on PCx activity. Results show that LEC inputs are heterogeneously distributed in the PCx microcircuit, evoking larger excitatory currents in SP and PV neurons due to higher monosynaptic connectivity. LEC inputs also differentially affect inhibitory circuits, activating PV while suppressing SOM interneurons. Studying the interaction between LEC inputs and sensory signals from the lateral olfactory tract (LOT) revealed that simultaneous LEC and LOT activation increases spiking in SP and DP neurons, with DP neurons showing a sharpened response due to LEC-induced inhibition that suppresses delayed LOT-evoked spikes. This suggests a regulatory mechanism where LEC inputs inhibit recurrent activity by activating PV interneurons. Our findings demonstrate that LEC afferents reconfigure PCx activity, aiding the understanding of how odor objects form within the PCx by integrating olfactory and nonolfactory information.

Embryonically active piriform cortex neurons promote intracortical recurrent connectivity during development

Neuronal activity plays a critical role in the maturation of circuits that propagate sensory information into the brain. How widely does early activity regulate circuit maturation across the developing brain? Here, we used targeted recombination in active populations (TRAP) to perform a brain-wide survey for prenatally active neurons in mice and identified the piriform cortex as an abundantly TRAPed region. Whole-cell recordings in neonatal slices revealed preferential interconnectivity within embryonically TRAPed piriform neurons and their enhanced synaptic connectivity with other piriform neurons. In vivo Neuropixels recordings in neonates demonstrated that embryonically TRAPed piriform neurons exhibit broad functional connectivity within piriform and lead spontaneous synchronized population activity during a transient neonatal period, when recurrent connectivity is strengthening. Selectively activating or silencing these neurons in neonates enhanced or suppressed recurrent synaptic strength, respectively. Thus, embryonically TRAPed piriform neurons represent an interconnected hub-like population whose activity promotes recurrent connectivity in early development.

Synaptic Connectivity and Electrophysiological Properties of the Nucleus of the Lateral Olfactory Tract

The sense of smell is tightly linked to emotions, a link that is thought to rely on the direct synaptic connections between the olfactory bulb (OB) and nuclei of the amygdala. However, there are multiple pathways projecting olfactory information to the amygdala, and their unique functions are unknown. The pathway via the nucleus of the lateral olfactory tract (NLOT) that receives input from olfactory regions and projects to the basolateral amygdala (BLA) is among them. NLOT has been very little studied, and consequentially its function is unknown. Furthermore, formulation of informed hypotheses about NLOT function is at this stage limited by the lack of knowledge about its connectivity and physiological properties. Here, we used virus-based tracing methods to systematically reveal inputs into NLOT, as well as NLOT projection targets in mice of both sexes. We found that the NLOT is interconnected with several olfactory brain regions and with the BLA. Some of these connections were reciprocal, and some showed unique interhemispheric patterns. We tested the excitable properties of NLOT neurons and the properties of each of the major synaptic inputs. We found that the NLOT receives powerful input from the piriform cortex, tenia tecta, and the BLA but only very weak input from the OB. When input crosses threshold, NLOT neurons respond with calcium-dependent bursts of action potentials. We hypothesize that this integration of olfactory and amygdalar inputs serves behaviors that combine smell and emotion.

Embryonically Active Piriform Cortex Neurons Promote Intracortical Recurrent Connectivity during Development

Neuronal activity plays a critical role in the maturation of circuits that propagate sensory information into the brain. How widely does early activity regulate circuit maturation across the developing brain? Here, we used Targeted Recombination in Active Populations (TRAP) to perform a brain-wide survey for prenatally active neurons in mice and identified the piriform cortex as an abundantly TRAPed region. Whole-cell recordings in neonatal slices revealed preferential interconnectivity within embryonically TRAPed piriform neurons and their enhanced synaptic connectivity with other piriform neurons. In vivo Neuropixels recordings in neonates demonstrated that embryonically TRAPed piriform neurons exhibit broad functional connectivity within piriform and lead spontaneous synchronized population activity during a transient neonatal period, when recurrent connectivity is strengthening. Selectively activating or silencing of these neurons in neonates enhanced or suppressed recurrent synaptic strength, respectively. Thus, embryonically TRAPed piriform neurons represent an interconnected hub-like population whose activity promotes recurrent connectivity in early development.

Distinct information conveyed to the olfactory bulb by feedforward input from the nose and feedback from the cortex

Sensory systems are organized hierarchically, but feedback projections frequently disrupt this order. In the olfactory bulb (OB), cortical feedback projections numerically match sensory inputs. To unravel information carried by these two streams, we imaged the activity of olfactory sensory neurons (OSNs) and cortical axons in the mouse OB using calcium indicators, multiphoton microscopy, and diverse olfactory stimuli. Here, we show that odorant mixtures of increasing complexity evoke progressively denser OSN activity, yet cortical feedback activity is of similar sparsity for all stimuli. Also, representations of complex mixtures are similar in OSNs but are decorrelated in cortical axons. While OSN responses to increasing odorant concentrations exhibit a sigmoidal relationship, cortical axonal responses are complex and nonmonotonic, which can be explained by a model with activity-dependent feedback inhibition in the cortex. Our study indicates that early-stage olfactory circuits have access to local feedforward signals and global, efficiently formatted information about odor scenes through cortical feedback.

Learning-dependent structural plasticity of intracortical and sensory connections to functional domains of the olfactory tubercle

The olfactory tubercle (OT), which is a component of the olfactory cortex and ventral striatum, has functional domains that play a role in odor-guided motivated behaviors. Learning odor-guided attractive and aversive behavior activates the anteromedial (am) and lateral (l) domains of the OT, respectively. However, the mechanism driving learning-dependent activation of specific OT domains remains unknown. We hypothesized that the neuronal connectivity of OT domains is plastically altered through olfactory experience. To examine the plastic potential of synaptic connections to OT domains, we optogenetically stimulated intracortical inputs from the piriform cortex or sensory inputs from the olfactory bulb to the OT in mice in association with a food reward for attractive learning and electrical foot shock for aversive learning. For both intracortical and sensory connections, axon boutons that terminated in the OT domains were larger in the amOT than in the lOT for mice exhibiting attractive learning and larger in the lOT than in the amOT for mice exhibiting aversive learning. These results indicate that both intracortical and sensory connections to the OT domains have learning-dependent plastic potential, suggesting that this plasticity underlies learning-dependent activation of specific OT domains and the acquisition of appropriate motivated behaviors.

Acute Fasting Modulates Food-Seeking Behavior and Neural Signaling in the Piriform Cortex

It is well known that the state of hunger can modulate hormones and hypothalamic neural circuits to drive food-seeking behavior and consumption. However, the role the sensory cortex plays in regulating foraging is much less explored. Here, we investigated whether acute fasting in mice can alter an odor-guided foraging behavior and how it can alter neurons and synapses in the (olfactory) piriform cortex (PC). Acute hunger enhances the motivation of a mouse to search for food pellets and increases food intake. The foraging behavior strongly activates the PC, as revealed by c-Fos immunostaining. The activation of PC is accompanied by an increase in excitation-inhibition ratio of synaptic density. Fasting also enhances the phosphorylation of AMP kinase, a biochemical energy regulator. Taken together, our results uncover a new regulatory brain region and implicate the PC in controlling foraging behavior.

Parallel processing by distinct classes of principal neurons in the olfactory cortex

Understanding how distinct neuron types in a neural circuit process and propagate information is essential for understanding what the circuit does and how it does it. The olfactory (piriform, PCx) cortex contains two main types of principal neurons, semilunar (SL) and superficial pyramidal (PYR) cells. SLs and PYRs have distinct morphologies, local connectivity, biophysical properties, and downstream projection targets. Odor processing in PCx is thought to occur in two sequential stages. First, SLs receive and integrate olfactory bulb input and then PYRs receive, transform, and transmit SL input. To test this model, we recorded from populations of optogenetically identified SLs and PYRs in awake, head-fixed mice. Notably, silencing SLs did not alter PYR odor responses, and SLs and PYRs exhibited differences in odor tuning properties and response discriminability that were consistent with their distinct embeddings within a sensory-associative cortex. Our results therefore suggest that SLs and PYRs form parallel channels for differentially processing odor information in and through PCx.

Target-specific control of piriform cortical output via distinct inhibitory circuits

Information represented by principal neurons in anterior piriform cortex (APC) is regulated by local, recurrent excitation and inhibition, but the circuit mechanisms remain elusive. Two types of layer 2 (L2) principal neurons, semilunar (SL), and superficial pyramidal (SP) cells, are parallel output channels, and the control of their activity gates the output of APC. Here, we examined the hypothesis that recurrent inhibition differentially regulates SL and SP cells. Patterned optogenetic stimulation revealed that the strength of recurrent inhibition is target- and layer-specific: L1 > L3 for SL cells, but L3 > L1 for SP cells. This target- and layer-specific inhibition was largely attributable to the parvalbumin (PV), but not somatostatin, interneurons. Intriguingly, olfactory experience selectively modulated the PV to SP microcircuit while maintaining the overall target and laminar specificity of inhibition. Together, these results indicate the importance of target-specific inhibitory wiring for odor processing, implicating these mechanisms in gating the output of piriform cortex.

Cell assembly formation and structure in a piriform cortex model

The piriform cortex is rich in recurrent excitatory synaptic connections between pyramidal neurons. We asked how such connections could shape cortical responses to olfactory lateral olfactory tract (LOT) inputs. For this, we constructed a computational network model of anterior piriform cortex with 2000 multicompartment, multiconductance neurons (500 semilunar, 1000 layer 2 and 500 layer 3 pyramids; 200 superficial interneurons of two types; 500 deep interneurons of three types; 500 LOT afferents), incorporating published and unpublished data. With a given distribution of LOT firing patterns, and increasing the strength of recurrent excitation, a small number of firing patterns were observed in pyramidal cell networks: first, sparse firings; then temporally and spatially concentrated epochs of action potentials, wherein each neuron fires one or two spikes; then more synchronized events, associated with bursts of action potentials in some pyramidal neurons. We suggest that one function of anterior piriform cortex is to transform ongoing streams of input spikes into temporally focused spike patterns, called here "cell assemblies", that are salient for downstream projection areas.

Circuit-Specific Dendritic Development in the Piriform Cortex

Dendritic geometry is largely determined during postnatal development and has a substantial impact on neural function. In sensory processing, postnatal development of the dendritic tree is affected by two dominant circuit motifs, ascending sensory feedforward inputs and descending and local recurrent connections. In the three-layered anterior piriform cortex (aPCx), neurons in the sublayers 2a and 2b display vertical segregation of these two circuit motifs. Here, we combined electrophysiology, detailed morphometry, and Ca²⁺ imaging in acute mouse brain slices and modeling to study circuit-specific aspects of dendritic development. We observed that determination of branching complexity, dendritic length increases, and pruning occurred in distinct developmental phases. Layer 2a and layer 2b neurons displayed developmental phase-specific differences between their apical and basal dendritic trees related to differences in circuit incorporation. We further identified functional candidate mechanisms for circuit-specific differences in postnatal dendritic growth in sublayers 2a and 2b at the mesoscale and microscale levels. Already in the first postnatal week, functional connectivity of layer 2a and layer 2b neurons during early spontaneous network activity scales with differences in basal dendritic growth. During the early critical period of sensory plasticity in the piriform cortex, our data are consistent with a model that proposes a role for dendritic NMDA-spikes in selecting branches for survival during developmental pruning in apical dendrites. The different stages of the morphologic and functional developmental pattern differences between layer 2a and layer 2b neurons demonstrate the complex interplay between dendritic development and circuit specificity.

Top-down inputs drive neuronal network rewiring and context-enhanced sensory processing in olfaction

Much of the computational power of the mammalian brain arises from its extensive top-down projections. To enable neuron-specific information processing these projections have to be precisely targeted. How such a specific connectivity emerges and what functions it supports is still poorly understood. We addressed these questions in silico in the context of the profound structural plasticity of the olfactory system. At the core of this plasticity are the granule cells of the olfactory bulb, which integrate bottom-up sensory inputs and top-down inputs delivered by vast top-down projections from cortical and other brain areas. We developed a biophysically supported computational model for the rewiring of the top-down projections and the intra-bulbar network via adult neurogenesis. The model captures various previous physiological and behavioral observations and makes specific predictions for the cortico-bulbar network connectivity that is learned by odor exposure and environmental contexts. Specifically, it predicts that-after learning-the granule-cell receptive fields with respect to sensory and with respect to cortical inputs are highly correlated. This enables cortical cells that respond to a learned odor to enact disynaptic inhibitory control specifically of bulbar principal cells that respond to that odor. For this the reciprocal nature of the granule cell synapses with the principal cells is essential. Functionally, the model predicts context-enhanced stimulus discrimination in cluttered environments ('olfactory cocktail parties') and the ability of the system to adapt to its tasks by rapidly switching between different odor-processing modes. These predictions are experimentally testable. At the same time they provide guidance for future experiments aimed at unraveling the cortico-bulbar connectivity.

Sodium and potassium conductances in principal neurons of the mouse piriform cortex: a quantitative description

KEY POINTS

The primary olfactory (or piriform) cortex is a promising model system for understanding how the cerebral cortex processes sensory information, although an investigation of the piriform cortex is hindered by a lack of detailed information about the intrinsic electrical properties of its component neurons. In the present study, we quantify the properties of voltage-dependent sodium currents and voltage- and calcium-dependent potassium currents in two important classes of excitatory neurons in the main input layer of the piriform cortex. We identify several classes of these currents and show that their properties are similar to those found in better-studied cortical regions. Our detailed quantitative descriptions of these currents will be valuable to computational neuroscientists who aim to build models that explain how the piriform cortex encodes odours.

ABSTRACT

The primary olfactory cortex (or piriform cortex, PC) is an anatomically simple palaeocortex that is increasingly used as a model system for investigating cortical sensory processing. However, little information is available on the intrinsic electrical conductances in neurons of the PC, hampering efforts to build realistic computational models of this cortex. In the present study, we used nucleated macropatches and whole-cell recordings to rigorously quantify the biophysical properties of voltage-gated sodium (Na_V ), voltage-gated potassium (K_V ) and calcium-activated potassium (K_Ca ) conductances in two major classes of glutamatergic neurons in layer 2 of the PC, semilunar (SL) cells and superficial pyramidal (SP) cells. We found that SL and SP cells both express a fast-inactivating Na_V current, two types of K_V current (A-type and delayed rectifier-type) and three types of K_Ca current (fast-, medium- and slow-afterhyperpolarization currents). The kinetic and voltage-dependent properties of the Na_V and K_V conductances were, with some exceptions, identical in SL and SP cells and similar to those found in neocortical pyramidal neurons. The K_Ca conductances were also similar across the different types of neurons. Our results are summarized in a series of empirical equations that should prove useful to computational neuroscientists seeking to model the PC. More broadly, our findings indicate that, at the level of single-cell electrical properties, this palaeocortex is not so different from the neocortex, vindicating efforts to use the PC as a model of cortical sensory processing in general.

Pre- and Postsynaptic Activation of GABAB Receptors Modulates Principal Cell Excitation in the Piriform Cortex

The piriform cortex (PC), like other cortical regions, normally operates in a state of dynamic equilibrium between excitation and inhibition. Here we examined the roles played by pre- and postsynaptic GABA_B receptors in maintaining this equilibrium in the PC. Using whole-cell recordings in brain slices from the anterior PC of mice, we found that synaptic activation of postsynaptic GABA_B receptors hyperpolarized the two major classes of layer 2 principal neurons and reduced the intrinsic electrical excitability of these neurons. Presynaptic GABA_B receptors are expressed on the terminals of associational (intracortical) glutamatergic axons in the PC. Heterosynaptic activation of these receptors reduced excitatory associational inputs onto principal cells. Presynaptic GABA_B receptors are also expressed on the axons of GABAergic interneurons in the PC, and blockade of these autoreceptors enhanced inhibitory inputs onto principal cells. Hence, presynaptic GABA_B autoreceptors produce disinhibition of principal cells. To study the functional consequences of GABA_B activation in vivo, we used 2-photon calcium imaging to simultaneously monitor the activity of ~200 layer 2 neurons. Superfusion of the GABA_B agonist baclofen reduced spontaneous random firing but also promoted synchronous epileptiform activity. These findings suggest that, while GABA_B activation can dampen excitability by engaging pre- and postsynaptic GABA_B heteroreceptors on glutamatergic neurons, it can also promote excitability by disinhibiting principal cells by activating presynaptic GABA_B autoreceptors on interneurons. Thus, depending on the dynamic balance of hetero- and autoinhibition, GABA_B receptors can function as variable modulators of circuit excitability in the PC.

Distinct projection patterns of different classes of layer 2 principal neurons in the olfactory cortex

The broadly-distributed, non-topographic projections to and from the olfactory cortex may suggest a flat, non-hierarchical organization in odor information processing. Layer 2 principal neurons in the anterior piriform cortex (APC) can be divided into 2 subtypes: semilunar (SL) and superficial pyramidal (SP) cells. Although it is known that SL and SP cells receive differential inputs from the olfactory bulb (OB), little is known about their projections to other olfactory regions. Here, we examined axonal projections of SL and SP cells using a combination of mouse genetics and retrograde labeling. Retrograde tracing from the OB or posterior piriform cortex (PPC) showed that the APC projects to these brain regions mainly through layer 2b cells, and dual-labeling revealed many cells extending collaterals to both target regions. Furthermore, a transgenic mouse line specifically labeling SL cells showed that they send profuse axonal projections to olfactory cortical areas, but not to the OB. These findings support a model in which information flow from SL to SP cells and back to the OB is mediated by a hierarchical feedback circuit, whereas both SL and SP cells broadcast information to higher olfactory areas in a parallel manner.

Odor identity coding by distributed ensembles of neurons in the mouse olfactory cortex

Olfactory perception and behaviors critically depend on the ability to identify an odor across a wide range of concentrations. Here, we use calcium imaging to determine how odor identity is encoded in olfactory cortex. We find that, despite considerable trial-to-trial variability, odor identity can accurately be decoded from ensembles of co-active neurons that are distributed across piriform cortex without any apparent spatial organization. However, piriform response patterns change substantially over a 100-fold change in odor concentration, apparently degrading the population representation of odor identity. We show that this problem can be resolved by decoding odor identity from a subpopulation of concentration-invariant piriform neurons. These concentration-invariant neurons are overrepresented in piriform cortex but not in olfactory bulb mitral and tufted cells. We therefore propose that distinct perceptual features of odors are encoded in independent subnetworks of neurons in the olfactory cortex.

Development and Organization of the Evolutionarily Conserved Three-Layered Olfactory Cortex

The olfactory cortex is part of the mammalian cerebral cortex together with the neocortex and the hippocampus. It receives direct input from the olfactory bulbs and participates in odor discrimination, association, and learning (Bekkers and Suzuki, 2013). It is thought to be an evolutionarily conserved paleocortex, which shares common characteristics with the three-layered general cortex of reptiles (Aboitiz et al., 2002). The olfactory cortex has been studied as a “simple model” to address sensory processing, though little is known about its precise cell origin, diversity, and identity. While the development and the cellular diversity of the six-layered neocortex are increasingly understood, the olfactory cortex remains poorly documented in these aspects. Here is a review of current knowledge of the development and organization of the olfactory cortex, keeping the analogy with those of the neocortex. The comparison of olfactory cortex and neocortex will allow the opening of evolutionary perspectives on cortical development.

Optogenetic Mapping of Intracortical Circuits Originating from Semilunar Cells in the Piriform Cortex

Despite its comparatively simple trilaminar architecture, the primary olfactory (piriform) cortex of mammals is capable of performing sophisticated sensory processing, an ability that is thought to depend critically on its extensive associational (intracortical) excitatory circuits. Here, we used a novel transgenic mouse model and optogenetics to measure the connectivity of associational circuits that originate in semilunar (SL) cells in layer 2a of the anterior piriform cortex (aPC). We generated a mouse line (48L) in which channelrhodopsin-2 (ChR) could be selectively expressed in a subset of SL cells. Light-evoked excitatory postsynaptic currents (EPSCs) could be evoked in superficial pyramidal cells (17.4% of n = 86 neurons) and deep pyramidal cells (33.3%, n = 9) in the aPC, but never in ChR- SL cells (0%, n = 34). Thus, SL cells monosynaptically excite pyramidal cells, but not other SL cells. Light-evoked EPSCs were also selectively elicited in 3 classes of GABAergic interneurons in layer 3 of the aPC. Our results show that SL cells are specialized for providing feedforward excitation of specific classes of neurons in the aPC, confirming that SL cells comprise a functionally distinctive input layer.

Balanced feedforward inhibition and dominant recurrent inhibition in olfactory cortex

Throughout the brain, the recruitment of feedforward and recurrent inhibition shapes neural responses. However, disentangling the relative contributions of these often-overlapping cortical circuits is challenging. The piriform cortex provides an ideal system to address this issue because the interneurons responsible for feedforward and recurrent inhibition are anatomically segregated in layer (L) 1 and L2/3 respectively. Here we use a combination of optical and electrical activation of interneurons to profile the inhibitory input received by three classes of principal excitatory neuron in the anterior piriform cortex. In all classes, we find that L1 interneurons provide weaker inhibition than L2/3 interneurons. Nonetheless, feedforward inhibitory strength covaries with the amount of afferent excitation received by each class of principal neuron. In contrast, intracortical stimulation of L2/3 evokes strong inhibition that dominates recurrent excitation in all classes. Finally, we find that the relative contributions of feedforward and recurrent pathways differ between principal neuron classes. Specifically, L2 neurons receive more reliable afferent drive and less overall inhibition than L3 neurons. Alternatively, L3 neurons receive substantially more intracortical inhibition. These three features--balanced afferent drive, dominant recurrent inhibition, and differential recruitment by afferent vs. intracortical circuits, dependent on cell class--suggest mechanisms for olfactory processing that may extend to other sensory cortices.

Hypobaric Preconditioning Modifies Group I mGluRs Signaling in Brain Cortex

The study assessed involvement of Ca(2+) signaling mediated by the metabotropic glutamate receptors mGluR1/5 in brain tolerance induced by hypoxic preconditioning. Acute slices of rat piriform cortex were tested 1 day after exposure of adult rats to mild hypobaric hypoxia for 2 h at a pressure of 480 hPa once a day for three consecutive days. We detected 44.1 ± 11.6 % suppression of in vitro anoxia-induced increases of intracellular Ca(2+) levels and a fivefold increase in Ca(2+) transients evoked by selective mGluR1/5 agonist, DHPG. Western blot analysis of cortical homogenates demonstrated a 11 ± 4 % decrease in mGluR1 immunoreactivity (IR), and in the nuclei-enriched fraction a 12 ± 3 % increase in IR of phospholipase Cβ1 (PLCβ1), which is a major mediator of mGluR1/5 signaling. Immunocytochemical analysis of the cortex revealed increase in the mGluR1/5 and PLCβ1 IR in perikarya, and a decrease in IR of the neuronal inositol trisphosphate receptors (IP3Rs). We suggest that enhanced expression of mGluR5 and PLCβ1 and potentiation of Ca(2+) signaling may represent pro-survival upregulation of Ca(2+)-dependent genomic processes, while decrease in mGluR1 and IP3R IR may be attributed to a feedback mechanism preventing excessive intracellular Ca(2+) release.

Looking for the roots of cortical sensory computation in three-layered cortices

Despite considerable effort over a century and the benefit of remarkable technical advances in the past few decades, we are still far from understanding mammalian cerebral neocortex. With its six layers, modular architecture, canonical circuits, innumerable cell types, and computational complexity, isocortex remains a challenging mystery. In this review, we argue that identifying the structural and functional similarities between mammalian piriform cortex and reptilian dorsal cortex could help reveal common organizational and computational principles and by extension, some of the most primordial computations carried out in cortical networks.

Thorough GABAergic innervation of the entire axon initial segment revealed by an optogenetic 'laserspritzer'

GABAergic terminals of chandelier cells exclusively innervate the axon initial segment (AIS) of excitatory neurons. Although the anatomy of these synapses has been well-studied in several brain areas, relatively little is known about their physiological properties. Using vesicular γ-aminobutyric acid transporter-channelrhodopsin 2-enhanced yellow fluorescence protein (VGAT-ChR2-YFP)-expressing mice and a novel fibreoptic 'laserspritzer' approach that we developed, we investigated the physiological properties of axo-axonic synapses (AASs) in brain slices from the piriform cortex (PC) of mice. AASs were in close proximity to voltage-gated Na(+) (NaV) channels located at the AIS. AASs were selectively activated by a 5 μm laserspritzer placed in close proximity to the AIS. Under a minimal laser stimulation condition and using whole-cell somatic voltage-clamp recordings, the amplitudes and kinetics of IPSCs mediated by AASs were similar to those mediated by perisomatic inhibitions. Results were further validated with channelrhodopsin 2-assisted circuit mapping (CRACM) of the entire inhibitory inputs map. For the first time, we revealed that the laserspritzer-induced AAS-IPSCs persisted in the presence of TTX and TEA but not 4-AP. Next, using gramicidin-based perforated patch recordings, we found that the GABA reversal potential (EGABA) was -73.6 ± 1.2 mV when induced at the AIS and -72.8 ± 1.1 mV when induced at the perisomatic site. Our anatomical and physiological results lead to the novel conclusions that: (1) AASs innervate the entire length of the AIS, as opposed to forming a highly concentrated cartridge, (2) AAS inhibition suppresses action potentials and epileptiform activity more robustly than perisomatic inhibitions, and (3) AAS activation alone can be sufficient to inhibit action potential generation and epileptiform activities in vitro.

Neurons and circuits for odor processing in the piriform cortex

Increased understanding of the early stages of olfaction has lead to a renewed interest in the higher brain regions responsible for forming unified 'odor images' from the chemical components detected by the nose. The piriform cortex, which is one of the first cortical destinations of olfactory information in mammals, is a primitive paleocortex that is critical for the synthetic perception of odors. Here we review recent work that examines the cellular neurophysiology of the piriform cortex. Exciting new findings have revealed how the neurons and circuits of the piriform cortex process odor information, demonstrating that, despite its superficial simplicity, the piriform cortex is a remarkably subtle and intricate neural circuit.

Input-specific excitation of olfactory cortex microcircuits

Every higher-order association cortex receives a variety of synaptic signals from different regions of the brain. How these cortical networks are capable of differentially responding to these various extrinsic synaptic inputs remains unclear. To address this issue, we studied how the basolateral amygdala (BLA) and the anterior piriform cortex (aPC) were functionally connected to the association olfactory cortex, the posterior piriform cortex (pPC). We infected the BLA and aPC with adeno-associated virus expressing channelrhodopsin-2-Venus fusion protein (ChR2-AAV) and recorded the excitatory postsynaptic currents (EPSC) resulting from photostimulation of either BLA or aPC axons in the major classes of excitatory and inhibitory neurons of the pPC. We found that BLA and aPC axons evoked monosynaptic EPSCs in every type of pPC neuron, but each fiber system preferentially targeted one excitatory and one inhibitory neuronal subtype. BLA fibers were most strongly connected to deep pyramidal cells (DP) and fast-spiking interneurons (FS), while aPC axons formed the strongest synaptic connections with DPs and irregular-spiking interneurons (IR). Overall, our findings show that the pPC differentially responds to amygdaloid versus cortical inputs by utilizing distinct local microcircuits, each defined by one predominant interneuronal subtype: FS for the BLA and IR for the aPC. It would thus seem that preferential excitation of a single neuronal class could be sufficient for the pPC to generate unique electrophysiological outputs in response to divergent synaptic input sources.

Optophysiological analysis of associational circuits in the olfactory cortex

Primary olfactory cortical areas receive direct input from the olfactory bulb, but also have extensive associational connections that have been mainly studied with classical anatomical methods. Here, we shed light on the functional properties of associational connections in the anterior and posterior piriform cortices (aPC and pPC) using optophysiological methods. We found that the aPC receives dense functional connections from the anterior olfactory nucleus (AON), a major hub in olfactory cortical circuits. The local recurrent connectivity within the aPC, long invoked in cortical autoassociative models, is sparse and weak. By contrast, the pPC receives negligible input from the AON, but has dense connections from the aPC as well as more local recurrent connections than the aPC. Finally, there are negligible functional connections from the pPC to aPC. Our study provides a circuit basis for a more sensory role for the aPC in odor processing and an associative role for the pPC.

Microcircuits mediating feedforward and feedback synaptic inhibition in the piriform cortex

Local inhibition by GABA-releasing neurons is important for the operation of sensory cortices, but the details of these inhibitory circuits remain unclear. We addressed this question in the olfactory system by making targeted recordings from identified classes of inhibitory and glutamatergic neurons in the piriform cortex (PC) of mice. First, we looked for feedforward synaptic inhibition provided by interneurons located in the outermost layer of the PC, layer Ia, which is the unique recipient of afferent fibers from the olfactory bulb. We found two types of feedforward inhibition: a fast-rising, spatially restricted kind that was generated by horizontal cells, and a slow-rising, more diffuse kind generated by neurogliaform cells. Both cell types targeted the distal apical dendrites of layer II principal neurons. Next, we studied feedback synaptic inhibition in isolation by making a tissue cut across layer I to selectively remove feedforward inhibitory connections. We identified a powerful type of feedback inhibition of layer II neurons, mostly generated by soma-targeting fast-spiking multipolar cells in layer III, which in turn were driven by feedforward excitation from layer II semilunar cells. Dynamic clamp simulation of feedback inhibition revealed differential effects of this inhibition on the two main types of layer II principal neurons. Thus, our results articulate the connectivity and functions of two important classes of inhibitory microcircuits in the PC. Feedforward and feedback inhibition generated by these circuits is likely to be required for the operation of this sensory paleocortex during the processing of olfactory information.

Microcircuits mediating feedforward and feedback synaptic inhibition in the piriform cortex

Local inhibition by GABA-releasing neurons is important for the operation of sensory cortices, but the details of these inhibitory circuits remain unclear. We addressed this question in the olfactory system by making targeted recordings from identified classes of inhibitory and glutamatergic neurons in the piriform cortex (PC) of mice. First, we looked for feedforward synaptic inhibition provided by interneurons located in the outermost layer of the PC, layer Ia, which is the unique recipient of afferent fibers from the olfactory bulb. We found two types of feedforward inhibition: a fast-rising, spatially restricted kind that was generated by horizontal cells, and a slow-rising, more diffuse kind generated by neurogliaform cells. Both cell types targeted the distal apical dendrites of layer II principal neurons. Next, we studied feedback synaptic inhibition in isolation by making a tissue cut across layer I to selectively remove feedforward inhibitory connections. We identified a powerful type of feedback inhibition of layer II neurons, mostly generated by soma-targeting fast-spiking multipolar cells in layer III, which in turn were driven by feedforward excitation from layer II semilunar cells. Dynamic clamp simulation of feedback inhibition revealed differential effects of this inhibition on the two main types of layer II principal neurons. Thus, our results articulate the connectivity and functions of two important classes of inhibitory microcircuits in the PC. Feedforward and feedback inhibition generated by these circuits is likely to be required for the operation of this sensory paleocortex during the processing of olfactory information.